Any student of chemistry who had the privilege of having access to
a Gilbert chemistry set would recall with nostalgia the pleasures of
experimenting with disappearing inks.

Let me commence this essay by illustrating a few general
demonstrative experiments that have the halo of chemical magic [1,2]:

* (1) You place a blank cardboard on a flame and mysteriously there
appears a black lettered message.

* (2) You heat a pink colored solution and suddenly the color
vanishes; on cooling the solution the color reappears and the procedure
can be repeated.

* (3) Again, you place a blank paper on a hot plate and there
appear the blue words of a love letter.

Now let me uncover the shroud of mystery to throw light into the
chemical facts behind these demonstrations as David Copperfield exposes
secrets behind his famous magical items in popular TV shows.

In the first one, the magician had written the message with
concentrated sulfuric acid.

On heating the cardboard that is made up of cellulose (a
carbohydrate, [([C.sub.6][H.sub.10][O.sub.5]).sub.n]), the acid removes
elements of water from it, leaving behind black carbon.

In the second, the pink color is due to the presence of
phenolphthalein in ammonium hydroxide. On heating, a shift of
equilibrium between ionized ammonium hydroxide and unionized ammonia
occurs, causing change in the color of the indicator.

In the third, the letter was written with cobaltous chloride
solution that was invisible on drying. This turns into blue on heating.

The chemistry as exemplified in the above instances may profitably
be utilized to prepare a class of inks called thermochromic inks.

Thermochromism

Thermochromism refers to the phenomenon of color changes by the
agency of heat. Obviously, the color changes are made possible by the
temperature-induced chemical or physical changes of materials
incorporated into the inks.

Sometimes, the color change occurring at a temperature is
permanent, and at other times the original color can be regained on
cooling.

Accordingly, we have an irreversible or reversible thermochromic
system. The required chemistry can be adopted based on the end use. That
means one can select an irreversible thermochromic system when a certain
temperature crossing is to be monitored and a reversible system when the
actual temperature range is to be monitored. The color change may be
achieved with a single chemical material or a mixture of them through
physical or chemical changes [3,4,5].

In fact, thermochromism is a special case of the phenomenon called
"chromotropism," which refers to the changes in color caused
by external influences. To this category belongs the phenomenon of
"piezochromism," which is the change in color caused by
pressure. If the color change is due to the frictional force, it is
referred to as "tribochromism."

The color changes observed when certain materials are ground in a
mortar come under the purview of this class, though the possibility of
color change emanating from a reduction in the particle size during
grinding should be ruled out.

Similarly, the color change shown in different solvents is called
as "solvatochromism." Added to this is the input from the
branch of photochemistry called photochromism that represents light
induced color transitions. Other areas such as
"electrochromism" (color changes caused by electricity) are
also emerging.

Applications

There are many applications where the temperature at which a
certain change occurs is required to be registered. For example, it may
be necessary to ensure that a delicate material or foodstuff is
maintained at a stipulated temperature range or a process does not
exceed certain temperature. It is not always convenient to monitor the
temperature variations in a system directly, say by the use of a
thermometer or a thermocouple device. Such a situation may arise even in
high-tech applications like computers where the microchip or the printed
circuit board (PCB) should not be allowed to surpass ambient
temperatures during production or use.

Thermochromic ink chemistry comes to the rescue in such instances.
An efficiently designed ink coating on the PCB can indicate the
temperature or temperature profile by showing remarkable color changes
in the coating at the transition temperature.

Thermochromic materials may either be inorganic or organic in
nature. Most of the early thermochromic chemicals were of inorganic type
and a wealth of literature is available on them. However, in modern
times, organic thermochromic systems are gaining popularity owing to the
vast strides in organic structure design.

A classical example of inorganic thermochromism is the
temperature-induced transition between monomeric nitrogen peroxide
([NO.sub.2]) and dimeric nitrogen tetroxide ([N.sub.2][O.sub.4]). When a
sealed tube containing brown [NO.sub.2] gas is cooled in ice, the color
fades away owing to the formation of the dimer [N.sub.2][O.sub.4] [6].

2 [NO.sub.2] [left arrow]right arrow] [N.sub.2][O.sub.4]

(Brown)

(Colorless)

Due to structural and electronic reasons, the absorption spectrum of the dimer differs drastically from that of the monomeric species. A
common reason for color is the absorption of light frequencies by the
molecule in the visible and/or near ultraviolet (UV) region of the
spectrum, though there are dozens of other minor and major reasons
responsible for it.

When light is absorbed, electrons in the molecule rearrange within
different energy levels facilitating the absorption process. Usually,
these electrons will be distributed in the molecule in locations called
orbitals designated as bonding, antibonding and nonbonding orbitals.

In the present case, the dimer absorbs light in the mid-UV region
rendering it colorless, and the monomer absorbs in the visible region
causing it to be colored.

The possibility of easy interconversion between the two species
coupled with the spectral shifts makes them thermochromic.

CT Complex Formation

An important thermochromic mechanism operating in solutions of a
simple molecule like iodine in various solvents is referred to as
"Charge-transfer" (CT) complex formation [7]. It is a common
observation that iodine shows a violet color in non-polar solvents such
as hexane, carbon tetrachloride, carbon disulfide, etc., and a brown
color in polar solvents such as acetone, alcohol, pyridine, etc.

The origin of these color differences is manifested in their
absorption spectra.

Figure 1 depicts the absorption spectra of iodine in five different
solvents with widely differing dielectric constant values, an index of
polarity of molecules [8]. Shifts in the absorption bands in the visible
and UV regions are discernible in these spectra. The change from the
conspicuous violet to brown is attributed to the CT complex formation
between the solvent and iodine.

This effect can be made clear as follows. A solvent (D:) like ether
which can donate a lone pair of nonbonding electrons to iodine can form
a CT complex by the formation of a coordinate bond between D: and iodine
leading to an oscillation (resonance) between the two structures shown
below.

The symbols [less than]-[greater than] and -[cdots] in the above
scheme respectively represent the resonance between the two structures
and coordination through electron pair donation from the solvent. In
aromatic solvents such as benzene and pyridine, the [phi] electron cloud
causing aromaticity in them is partially transferred to iodine.

The above principle may also be used to explain the thermochromism
of iodine in solvents such as ethyl stearate. Iodine in ethyl stearate
is brown at room temperature which on heating above 80[degrees]C becomes
violet. At room temperature, the polar ends of the long chain molecules
form CT complexes with iodine.

On heating, this CT complex dissociates, and the iodine thus made
free will be surrounded by the long chain hydrocarbon environment
provided by the stearyl group. This situation gives the typical
aliphatic hydrocarbon surroundings to iodine where it is shown to be
violet.

Reversible Thermochromism

Another major mechanism behind thermochromic effect in inorganic
compounds can be illustrated with the example of ruby, the red colored
gem.

Ruby shows reversible thermochromism changing from red to green
through violet on heating. [9] Chemically speaking, ruby is a solid
solution of chromium and aluminum oxides. The color change in it is
explained with the help of ligand field theory of transition metal
complexes. Chromium is a transition metal characterized by d orbitals.
The color of transition metal compounds is due to the electronic
transition within d orbitals generally called as d-d transitions.

These d orbitals split in terms of energy under the influence of a
ligand. Ruby's normal red color is due to the unusually large
splitting of the d orbitals caused by the ligand field. On heating, the
crystal structure expands, and the [Cr.sup.3+] ions become more relaxed
and regain its original green color.

The ligand field splitting theory can explain the thermochromism in
other transition metal compounds also. The hydrated green salts of
nickelion ([Ni.sup.2+]), on cooling in liquid nitrogen (-196[degrees]C),
acquires a sky blue color. [10] Similarly, the red salts of cobalt ion
changes to yellow at liquid nitrogen temperature. The Werner's
coordination complexes of nickel and cobalt ions with a coordination
number of six represented by [[Ni([[H.sub.2]O).sub.6]].sup.2+] and
[[Co([[H.sub.2]O).sub.6].sup.2+] undergo strong contraction at
increasing ligand fields.

Sometimes, a mere phase transition can result in color change of
certain solids. A case in point is that of mercuric iodide, which is red
at normal temperatures. Above 127[degrees]C, it exhibits a reversible
thermochromism turning into yellow. Here a red tetragonal phase is
transformed into a yellow rhombic phase. The mechanism responsible for
color changes is a variation of CT process that operates by donating
electrons from the ligand to metal. The thermochromism in salts like
silver iodide is also explained in similar lines. The same mechanism
explains the thermochromism in certain double salts as illustrated by
the following examples. [11] [Ag.sub.2][HgI.sub.4], a double salt of
mercuric and silver iodides, shows thermochromism at 50.7[degrees]C
changing from yellow to orange. Similarly, [Cu.sub.2][HgI.sub.4] turns
from red to black at 67[degrees]C.

Sone and Fukuda [12] have compiled the names of important
thermochromic systems, both reversible and irreversible, in their
monograph on this topic. Interestingly enough, most of these examples
utilize the color bearing chemistry of first transition metals such as
chromium, vanadium, cobalt and nickel.

The multimillion numbers of organic compounds and polymers can
provide many useful examples of thermochromism [3,13]. Normal organic
compounds are colorless since their absorption bands lie in the UV
region. But factors like conjugated double bonds shift the absorption to
the visible region imparting color. Structural factors within the
molecule that contribute to the color forming mechanism can be altered
by temperature variations, and such molecules can be efficiently
designed due to the versatility of organic chemical methods.

Specifically, the CT energy levels are altered when the polymer
undergoes hydration and dehydration processes as a function of
temperature.

A hybrid approach assimilating the potentials of inorganic and
organic chemistry is also being tried to evolve novel thermochromic
systems. For example, Takeda et al. [14] investigated recently the color
of polycrystals of the zirconocene complex with
1,4-diphenyl-1,3-butadiene. They performed nuclear magnetic resonance cross polarization/magic angle spinning studies, quasielastic neutron
scattering experiments, molecular orbital calculations, etc. in an
attempt to reveal the minute details of the molecular transitions in
this complex subsequent to temperature variations.

Liquid Crystals

In the recent past, thermochromism has been achieved by the
temperature-induced rearrangement of liquid crystals [15].

Liquid crystals are phases exhibiting properties in between liquids
and solids. They belong to the two general classes referred to as
thermotropic and lyotropic liquid crystals. Thermotropic liquid crystals
are formed by heating solids and lyotropic liquid crystals are formed by
dissolving solids in liquids.

Liquid crystals that show thermochromism are made by
microencapsulating cholesteryl ester-based compounds. The color changes
in them can be reversed facilitating temperature detection. These
chemicals are thick fluids whose molecules spontaneously align. They
form a helically twisted structure like the strands of a rope. With
temperature, the pitch of the helix changes resulting in differential
reflection leading to color variation. The microencapsulation protects
the pure chemicals from contamination.

Finally, irreversible thermochromic systems may be designed based
on ninny chemical reactions using multiple components that lead to
colored end products on heating.

One can apply the immense potential derived from inorganic
qualitative analysis of cationic radicals where many colored salts are
formed in the course of experimentation, such as the colorful sulfides
of various metals. What is needed is to arrive at a system where these
salts may be formed from the component reagents on heating. In this
respect, organic chemistry is more resourceful since it is replete with
numerous classes of compounds and reactions from which an appropriate
thermal reaction yielding colored end product may be identified.

They are incorporated in inks, paints, crayons, adhesive labels,
etc. to mark the temperature variation in a wide range. As implied in
the beginning of this essay, thermochromic inks and paints can display
the temperature in a real time basis on many moving parts of instruments
instantly, and these colors can even be recorded.

Ink manufacturers have been successful in translating the chemical
knowledge into technology with projected new applications on the horizon
such as security inks [16], and established applications such as
sterilization indicators.

This branch of technology will be more colorful with the
possibility of fusing concepts such as photochromism with thermochromism
in a general ink system [17,18].

Joy T. Kunjappu received his Ph.D. inorganic photochemistry in 1985
and D.Sc. in physical chemistry of surfactants in 1996. Prior to his
arrival to the U.S. in 1987, he served as a senior scientific officer
with the Department of Atomic Energy of India specializing on many
aspects of chemistry. He worked as a post-doctoral research scientist
(1987-1989) and associate research scientist (1994-1996) at the Langmuir
Center for Colloids and Interfaces and the Chemistry Department of
Columbia University, New York.

He has authored about 70 publications which include original
research papers, review articles, book chapters book reviews and
symposium proceedings. He also served as the reviewer of technical and
scientific papers of eight international publications. In 1989, he
edited a special issue of Colloids and Surfaces (Aspects of Interfaces)
as a guest editor. His biography is featured in "Marquis Who's
Who in Science and Engineering" (1997) Currently, Dr. Kunjappu is
working as a research chemist at Propper M.C., Inc New York. He may be
reached at jkunjappu@ aol.com and (212) 942-4828.